If this is only a single group of 'key', one option would be to use base R. We transpose (t) the dataset except the first column (df[-1]), concatenate to a vector, convert to data.frme and cbind with the first element of the first column.
res <- cbind(df1[1,1], as.data.frame.list(c(t(df1[-1]))))
names(res) <- c(names(df1)[1] ,make.unique(rep(names(df1)[-1],3)))
res
# Key A B C A.1 B.1 C.1 A.2 B.2 C.2
#1 1 1 2 3 4 6 8 3 2 1
Or use data.table i.e. the development version v1.9.7 if there are many 'Key' elements
library(data.table)
dcast(setDT(df1), Key~rowid(Key), value.var = c("A", "B", "C"),sep="")
# Key A1 A2 A3 B1 B2 B3 C1 C2 C3
#1: 1 1 4 3 2 6 2 3 8 1
if we have a data.table with version < v1.9.7, we need to create the sequence column grouped by 'Key'.
dcast(setDT(df1)[, rn := 1:.N , Key], Key ~rn, value.var = c("A", "B", "C"),sep="")
# Key A1 A2 A3 B1 B2 B3 C1 C2 C3
#1: 1 1 4 3 2 6 2 3 8 1
